1. Field of the Invention
The present invention relates to a method and apparatus for hydroforming a metallic tube in a which the metallic tube is formed in a closed die cavity using pressurized fluid introduced into the metallic tube.
2. Description of the Related Art
Metallic-tube hydroforming comprises the steps of introducing a hydraulic fluid into a metallic tube serving as a material tube (hereinafter, referred to merely as a metallic tube) and applying an axial force to the tube ends, to thereby form the metallic tube through combined use of hydraulic pressure and the axial forces. The process provides tubular parts having a variety of cross-sectional shapes. FIG. 5 shows a metallic tube and a product, wherein FIG. 5(a) shows a longitudinal sectional view of a metallic tube 1 and FIG. 5(b) shows a partially sectional view of a product 2 obtained through hydroforming.
In the product of FIG. 5(b), radially expanded portion 2a having an outer diameter D and a length W.sub.1 is formed in the central section of the product, and tubular portions having the same outer diameter d as that of the metallic tube 1 of FIG. 5(a) (hereinafter referred to as straight portions) extend lengthwise from the expanded portion 2a. The overall length L.sub.1 of the product 2 becomes shorter than the length L.sub.0 of the metallic tube 1 because of axial pressing.
FIG. 6 shows a typical tooling used in a conventional hydroforming apparatus for obtaining the product 2, wherein FIG. 6(a) is a longitudinal sectional view and FIG. 6(b) is a sectional view taken along the line C--C in FIG. 6(a).
A tool 15 includes a die composed of a lower die 3 and an upper die 4 and left and right punches 5 and 6. The lower die 3 and the upper die 4 have tube-holding grooves 3a and 4a and die cavities 3b and 4b formed therein, respectively. The diameter d of the tube-holding grooves 3a and 4a is identical to the outer diameter of the metallic tube 1 The die cavities 3b and 4b define a space for forming the expanded portion of a product. The internal contour of the die cavities 3b and 4b is identical to the external contour of the expanded portion of a products. Die shoulders 3c and 4c have a radius of curvature equal to a forming corner radius r1 of the product shown in FIG. 5(b). An ejector 17 is mounted in a vertically slidable manner in the lower die 3 at the bottom of the die cavity 3b for the purpose of ejecting a formed product. The punches 5 and 6 have substantially the same diameter as the outer diameter d of a metallic tube and are provided with flanges 5C and 6C, respectively, at their outer ends for connection to axial pistons, which will be described later. The punch 5 has a through path 5b formed therein for introducing a hydraulic fluid, which will be described later, into a metallic tube, and the punch 6 has a through path 6b formed therein for ejecting air from the interior of the metallic tube.
FIG. 7 shows a process for hydroforming a metallic tube by applying an internal pressure and axial forces to the metallic tube through use of the tool 15, wherein FIG. 7(a) is a longitudinal sectional view showing a state immediately before hydroforming is started and FIG. 7(b) is a longitudinal sectional view showing a state when hydroforming is completed.
First, the metallic tube 1 is set in the lower die 3. The upper die 4, attached to a vertical press unit which will be described later, is lowered so as to press the lower die 3 with a predetermined force. Next, the punches 5 and 6, attached to respective horizontal press units which will be described later, are advanced from the left- and right-hand sides such that their top end portions 5a and 6a seal the corresponding ends of the metallic tube 1. While a hydraulic fluid 7 is being introduced into the metallic tube 1 through the path 5b in the left-hand punch 5, air inside the metallic tube 1 is ejected through the path 6b in the right-hand punch 6. Then, an unillustrated valve located on the extension of the path 6b is closed to thereby fill the interior of the metallic tube 1 with the hydraulic fluid 7. This state is shown in FIG. 7(a).
Next, the punches 5 and 6 are advanced from the left- and right-hand sides, and the internal pressure of the metallic tube 1 is gradually increased by means of an unillustrated pump. Thus, the material of the metallic tube 1 is expanded into the die cavities 3b and 4b to thereby form a product 2 as shown in FIG. 7(b). The internal pressure of the metallic tube 1 is gradually increased so as to expand the tube material which work-hardens gradually as it is pressed into the die cavities 3b and 4b through axial pressing. When the tube material has a high strength and a large wall thickness or when the forming corner radius of an expanded portion is small, a required internal pressure is high. Subsequently, the internal pressure is reduced, the upper die 4 is raised, the punches are retreated to drain the hydraulic fluid from inside the product 2, and the ejector 17 is raised to remove the product 2 from the lower die 3.
An example of the hydraulic fluid 7 is an emulsion in which a fat-and-oil component is uniformly dispersed in water in an amount of several percent so as to produce a rust-preventive effect.
Next will be described a conventional apparatus for performing the above hydroforming process.
FIG. 8 shows a conventional hydroforming apparatus, wherein FIG. 8(a) shows a front view of the apparatus and FIG. 8(b) shows a sectional view of a horizontal press unit.
As shown in FIG. 8(a), the hydroforming apparatus includes a vertical press unit 21 and horizontal press units 22 and 23. These press units share a bed 24. The vertical press unit 21 includes a frame 26 connected to the bed 24 by means of columns 25, a pressure cylinder 27 attached to the frame 26, a ram 28 of the cylinder 27, and a ram head 29 attached to the ram 28. The lower die 3 is removably mounted on the bed 24, and the upper die 4 is removably mounted on the ram head 29. A cylinder 19 is provided just under the lower die 3 in order to vertically move the ejector 17.
As shown in FIG. 8(b), the horizontal press unit 22 includes a cylinder case 30 and a piston 31. The punch 5 is removably attached to the tip portion 31d of the piston 31 through bolting or the like. The piston 31 has a hydraulic fluid path 31C formed therein and communicating with the path 5b formed in the punch 5. The hydraulic fluid path 31C communicates with an unillustrated external pump via a hollow beam 33 connected to the rear end of the piston 31 and via a piping 32. The piston 31 moves axially within the cylinder case 30 under the guidance established between the outer surface 31a of the piston 31 and a cylinder flange 30b, between a piston flange 31b and the shell 30a of the cylinder case 30, and between the hollow beam 33 and the rear plate 30c of the cylinder case 30.
Seals 40, 41, and 42 are provided in the above guide portions. When a hydraulic fluid having a predetermined pressure is fed into a rear pressure chamber 50 from an unillustrated external pump via a path 51 formed in the cylinder case 30 and a piping 52, the piston 31 advances. In contrast, when a hydraulic fluid having a predetermined pressure is fed into a front pressure chamber 60 from an unillustrated external pump via a path 61 formed in the cylinder case 30 and a piping 62, the piston 31 retreats.
The above hydroforming process involves the following problems.
A first problem relates to axial pressing. As mentioned previously, axial pressing and an internal pressure play an important role in hydroforming. Particularly, for a product which involves a large increase in circumferential length caused by expansion forming, an axial pressing plays a particularly important role. When an internal pressure is increased while axial pressing is insufficient, the wall thickness of a portion to be expanded decreases progressively, resulting in the rupture of the portion. In order to suppress a reduction of wall thickness, a tube material must be pressed into a die cavity by axial pressing before an internal pressure is increased, so as to form a raised portion having a double curved surface to thereby increase resistance to rupture.
In the hydroforming process described above with reference to FIG. 7(a), factors adverse to axial pressing are the following two: friction between a tube material and the tube-holding grooves 3a and 4a; and friction between the die shoulders 3c and 4c and a tube material sliding along the radii of the die shoulders 3c and 4c, and a bending deformation of a tube material sliding along the radii of the die shoulders 3c and 4c. The former factor relates to a coefficient of friction and the length l of a tube material in contact with the tube-holding grooves 3a and 4a. The latter factor relates to a coefficient of friction and the radius r1 of the die shoulders 3c and 4c (as the radius r1 decreases, resistance to axial pressing increases) if the strength of a tube material is not taken into consideration. In order to reduce a coefficient of friction, a hydroforming die is manufactured of a hard material so that the die becomes endurable to damage upon sliding contact with a tube material, and tube-holding grooves and die shoulders are finished smoothly. Also, to maintain the smoothly finished condition, the die surface must be polished regularly.
Further, in order to prevent seizure between a tube material and the die, in many cases the outer surface of the metallic tube 1 is coated with a lubricant or paint. However, even when such measures are employed, if the length l of a tube material in contact with the tube-holding grooves 3a and 4a (see FIG. 7(a)) is relatively large, the contact area between the tube material and the tube-holding grooves 3a and 4a become relatively large. Consequently, there becomes relatively large a frictional resistance associated with the movement of the entire tube material within the tube-holding grooves 3a and 4a.
FIG. 9 is a longitudinal sectional view showing the occurrence of a defect during hydroforming, wherein FIG. 9(a) shows the occurrence of buckling and FIG. 9(b) shows the occurrence of wall thickening at tube end portions.
In the case of a thin-walled tube, buckling as represented by reference numeral 8 in FIG. 9(a) is likely to occur at the straight portions during axial pressing. Accordingly, in the case of a thin-walled carbon steel tube, axial pressing becomes hard to perform at a l/d value of 2.0 or more for t/d=0.03 (t: wall thickness) and at a l/d value of 1.5 or more for t/d=0.02.
In the case of a thick-walled tube, buckling is less likely to occur. However, resistance to axial pressing increases due to an increase in resistance to bending along the radii of the die shoulders 3c and 4c. Consequently, as shown in FIG. 9(b), a thick-walled portion 9 thicker than the wall thickness of the metallic tube 1 is formed at tube end portions, thus hindering expansion. Accordingly, in order to form the expanded portion 2a having a predetermined shape, the overall amount of axial pressing (that is, the length of the metallic tube 1) must unavoidably be increased with a resultant deterioration in material yield. Also, in addition to an increase in product weight, the thick-walled portions 9 may need to be machined after hydroforming in order for the finish wall thickness to meet a predetermined value.
A second problem relates to the die manufacturing cost. The lengths (along the axial direction of the metallic tube 1) of the lower and upper dies 3 and 4, respectively, must be increased. That is, as shown in FIG. 7, the lower and upper dies 3 and 4, respectively, must have a length equivalent to the overall length of the metallic tube 1 plus the length of the tip portions of the punches 5 and 6 to be inserted into the lower and upper dies 3 and 4. As mentioned previously, in many cases the die is manufactured of a hard material in order to prevent seizure between a tube material and the die. Accordingly, an increase in die length causes an increase in material cost as well as an increase in man-hours for machining the tube-holding grooves 3a and 4a. Also, the die cavities 3b and 4b must be machined by an end mill in the lower and upper dies 3 and 4, respectively, according to the shape of the expanded portion 2a of a product, and the surfaces of the die cavities 3b and 4b must be finished smoothly. Thus, machining cost increases. Some shapes may be hard to machine and must unavoidably be formed through use of expensive electric discharge machining. Further, when products having expanded portions of different sizes are to be manufactured, dies having corresponding die cavities of different sizes must be prepared.
Further, since the die cavities 3b and 4b must have a shape enabling a product to be ejected therefrom, the shape of a certain expanded portion of a product may be hardly formed merely through use of the die cavities 3b and 4b.
FIG. 10 exemplifies a product having such a rather complex shaped expanded portion, wherein FIG. 10(a) is a longitudinal sectional view of a product 70 and FIG. 10(b) is a front view of the product 70. Indentations 70c are formed in the side walls of the expanded portion 70a of the product 70. Accordingly, if the internal contour of the die cavity is identical to the external contour of the expanded portion 70a, the formed product 70 cannot be ejected.
FIG. 11 is a longitudinal sectional view showing the structure of a die for manufacturing the product 70 of FIG. 10. As seen from FIG. 11, the product 70 is manufactured in the steps of: forming an expanded portion not having the indentations 70c; projecting punches 72 by means of pressure cylinders 71 built in lower and upper dies 3-1 and 4-1, respectively, so as to form the indentations 70c; retreating the punches 72; and removing the product 70 from the die. Thus, the die structure becomes complex, and manufacturing cost increases accordingly.